1. Field of the Invention
This invention relates to wireless communications equipment that performs conversion between baseband signals at a baseband frequency and intermediate frequency signals of a plurality of intermediate frequencies, and particularly to wireless communications equipment of Fixed Wireless Access (FWA) systems that improves the efficiency of a configuration that performs this conversion.
2. Description of the Prior Art
In FWA systems, for example, development is now proceeding rapidly on point-to-point and point-to-multipoint circuit configurations in subscriber wireless access systems that utilize the millimeter-wave band. Subscriber wireless access is also called Wireless Local Loop (WLL) or the like.
In addition, the Time Division Duplex (TDD) communications technique is used in mobile telephony and other systems, and is known to be a technique wherein sending and receiving are performed alternately in time division.
In addition, the next-generation broadband FWA systems consist of subscriber wireless equipment called nodes disposed in a lattice shape (mesh shape) in a city or the like. In addition, these systems are built using autonomous route selection technology that predicts the radio-wave propagation situation depending on the direction and strength of the radio waves received by each node and autonomously selects a channel and direction of radiation with no interference that is usable for transmission, technology that constantly monitors the usage situation and circuit quality of the links among nodes to select the optimal route among a plurality of routes, and wireless routine technology that selects the optimal route in a mesh network architecture for performing communications. This configuration is being adopted in order to construct novel FWA systems. In addition, in order to increase the information transmission speeds, the next-generation broadband FWA systems adopt multiplexing technology whereby, in communication among heavily loaded nodes, sending and receiving is performed by using a bunch of several wireless channels, of which a plurality are prepared on the frequency axis.
FIG. 7 presents an example of a configuration of a frequency conversion circuit in the case that a frequency conversion circuit is constructed in next-generation broadband FWA units using the prior art. The frequency conversion circuit shown in the figure consists of: a signal processor 41, band pass filter (BPF) 42, phase-locked loop (PLL) 43, mixer 44, amplifier (AMP) 45, single-pole six-throw (SP6T) switch 46, six surface acoustic wave (SAW) filters F1-F6, a SP6T switch 47, transmitter power controller 48, TDD switch 49, intermediate frequency (IF) signal input/output pin 50, amplifier 51, SP6T switch 52, six SAW filters F11-F16, a SP6T switch 53, mixer 54 and an automatic gain control (AGC) amplifier 55.
In addition, FIG. 8 shows an example of frequency conversion performed by the frequency conversion circuit shown in FIG. 7. In FIG. 8, the horizontal axis is the frequency and the vertical axis is the power. FIG. 8 shows the baseband signals at the baseband frequency f0, intermediate frequency signals at six intermediate frequencies f1-f6, signals at carrier leak frequencies L1-L6 and signals at image frequencies I1-I6 corresponding to each of the intermediate frequency signals. Note that for i=1-6, f1=f0+L1 and I1=L1−f0.
In the following description, we shall make reference to FIGS. 7 and 8 to illustrate one example of the signal processing performed by the frequency conversion circuit shown in FIG. 7.
The signal processor 41 performs digital quadrature modulation of the transmission signal to generate a quadrature modulated wave which has a relatively low frequency of f0.
In addition, the signal processor 41 uses a control signal to control the PLL 43, and the PLL 43 generates a signal at the controlled frequency (here, one of L1-L6) which is output to the two mixers 44 and 54.
In addition, the signal processor 41 uses control signals to control the four SP6T switches 46, 47, 52 and 53, transmitter power controller 48 and AGC amplifier 55, and also uses timing signals to control the TDD switch 49.
The send signals at the frequency of f0 generated by the signal processor 41 (here, the baseband signals) are subjected to removal of spurious emissions by the BPF 42 and then converted by the mixer 44 to an intermediate frequency signal at one of the frequencies f1-f6 which are the intermediate frequencies. Thereafter, the send signals converted to an intermediate frequency are amplified by the amplifier 45 and output to the SAW filter (one of F1-F6) corresponding to the intermediate frequency selected by the SP6T switch 46 which is switched under the control of the signal processor 41.
The six SAW filters F1-F6 correspond to each of the six central frequencies f1-f6, and if the send frequency (here, the central frequency) is f1, for example, the SAW filter F1 which has a central frequency of f1 is selected. Each of the SAW filters F1-F6 removes the signals at the carrier leak frequencies L1-L6 and the signals at the image frequencies I1-I6, which are spurious emissions in the send signals, and outputs the send signals with these signals removed to the transmitter power controller 48 via the SP6T switch 47.
The transmitter power controller 48 increases or decreases the transmitter power of the send signals under the control of the controller (here, the signal processor 41) and outputs the send signals after power control to the TDD switch 49. Under the control of the controller (signal processor 41), the TDD switch 49 switches the signal system between a system for sending and a system for receive depending on whether sending or receiving is to be done. At the time of sending, the TDD switch 49 is switched to the transmitter power controller 48 side and outputs the send signals to the IF signal input/output pin 50.
Note that when performing digital quadrature modulation, with the current level of devices, the limit from a cost and hardware standpoint lies at handling modulation waves below 50 MHz. This fact means that during sending, the local leak (carrier leak) frequency and image frequency are generated at relatively close areas detuned by less than 50 MHz from the central frequency of the main signal, so it is necessary to use a filter such as a SAW filler that has a steep skirt characteristic.
In addition, in the next-generation broadband FWA for example, the use of high order modulation (1024 QAM) is under consideration, and in this case, the conventional analog quadrature modulation methods do not give adequate modulation precision so interference between symbols occurs and practical application is difficult. For this reason, in the next-generation broadband FWA systems, it has been necessary to select digital quadrature modulation.
On the other hand, during receiving, the TDD switch 49 is switched to the amplifier 51 side and the received signal (here, the intermediate frequency signal) input from the IF signal input/output pin 50 is output to the receiving amplifier 51 via the TDD switch 49. This received signal is a signal that has one of the frequencics among the six central frequencies f1-f6, and after being amplified by the amplifier 51, in the same manner as on the sending side, under the control of the controller (here, the signal processor 41) for two SP6T switches 52 and 53, the signal is caused to pass through the SAW filter (one of F11-F16) for the appropriate frequency (one of f1-f6) and the spurious emissions and image signals are removed.
Thereafter, the received signal is converted by the mixer 54 to a baseband signal with a central frequency of f0 and output to the AGC amplifier 55. Based on control signals from the controller (here, the signal processor 41) the AGC amplifier 55 is subjected to feedback control such that the output power value is constantly kept at the target power value, and thus the baseband signal obtained from the received signal is output to the signal processor 41 at a constant power. The signal processor 41 subjects the received signal input as a baseband signal to digital quadrature detection, thereby demodulating it to the original information.
As indicated by the example of configuration and example of operation of the frequency conversion circuits of the next-generation broadband FWA units constituted using the prior art illustrated above, the input/output frequency f0 of the signal processor 41 becomes the baseband frequency during both sending and receiving. In addition, the send/receive frequency (here, the central frequency) is one of f1-f6 and if f, for example, by adding the oscillation frequency from PLL 43 as the frequency L1 (L1=f1−f0) and the signal of this frequency L1 as the local signal to mixers 44 and 54, during sending, the baseband signal with the baseband frequency f0 is converted to the intermediate frequency signal with the intermediate frequency f1, but during receiving, the intermediate frequency signal with the intermediate frequency f1 is converted to the baseband signal with the baseband frequency f0.
Note that a FWA unit is typically divided into the indoor unit (IDU) and outdoor unit (ODU), and the portion of the configuration shown in FIG. 7 corresponds to the portion of the configuration of the indoor unit. The IDU and ODU may be communicably connected by coaxial cable, for example, and the intermediate frequency used can be a low frequency in the 70 MHz band or 140 MHz band that is relatively unaffected by losses due to the coaxial cable, for example. In addition, the ODU performs frequency conversion between the intermediate frequency signals of the intermediate frequency and the signals in the radio frequency (RF) which is the millimeter-wave band, and emits radio waves from the antenna and receives radio waves with the antenna.
As an example, an FWA unit may consist of a base station unit and a subscriber station unit. In this case, the base station unit and subscriber station unit may each consist of an outdoor unit installed in a fixed location on the top of a building or tower or other high place and an indoor unit installed within the building or the like, and connected by cable. Moreover, the outdoor unit of the base station unit or subscriber station unit is equipped with a wireless processor that performs the processing of wireless communications using an antenna by mainly controlling the processing of wireless communications. In addition, the indoor unit of the base station unit mainly controls data communications with the backbone network, while the indoor unit of the subscriber station unit mainly controls data communications with personal computers or other communications terminal units or a LAN or the like.
In addition, for reference, we shall also introduce an example of prior art pertaining to frequency conversion.
The “transceiver for wireless communication equipment” recited in the publication of unexamined Japanese patent application JP-A-10-93476 uses a configuration provided with a local signal generation means with an n-stage configuration (n is an integer equal to 2 or greater) that supplies send local signals and receive local signals each synchronized to one of n contiguous slots in the TDMA-TDD protocol, along with a double superheterodyne configuration wherein frequency conversion is performed in two stages in the same direction of lowering the frequency, and a configuration wherein filters are shared between sending and receiving. Note that in comparison with the present invention to be presented below, the function of sharing filters between sending and receiving (e.g., frequency band) and its object differ from those of the present invention, and the object of two-stage frequency conversion differs from that of the present invention. This also differs from the present invention on the point that it does not have the configuration of mixers and filters and combination of mixers used in the present invention. In addition, this differs from the present invention on the point that it is thought to be difficult to apply to a FWA system.
The “wireless communication equipment” recited in the publication of unexamined Japanese patent application JP-A-5-129984 uses a configuration wherein filters and the like are shared between sending and receiving. Note that in comparison with the present invention to be presented below, the function of sharing filters between sending and receiving (e.g., frequency band) and its object differ from those of the present invention, and it also differs from the present invention on the point that it does not have the configuration of mixers and filters and combination of mixers used in the present invention.
However, with the configuration of the frequency conversion circuit for use in next-generation broadband FWA systems or the like as shown in FIG. 7 above, there is a cost-related problem in that the same number of SAW filters as the number of channels is required. Specifically, the example shown in FIG. 7 above shows the case of six channels, and in this case, six SAW filters are required for each of the sending system and the receiving system, thus causing the problem of higher cost and a larger unit. In addition, even though the configuration of frequency conversion is identical in the sending system and receiving system with the exception of the point that the direction of frequency conversion is different, it has two hardware systems, for the sending system and for the receiving system, so there is a problem in that hardware is redundant.
The present invention came about in order to solve the aforementioned problems in the prior art, and has as its object to provide wireless communication equipment with a more efficient configuration for performing the conversion between baseband signals on the baseband frequency and intermediate frequency signals on a plurality of intermediate frequencies, and also a FWA system configured using such wireless communication equipment.
More specifically, the present invention provides technology for reducing the number of SAW filters and other IF filters and technology for adopting common hardware for sending and receiving in the frequency conversion circuits of next-generation broadband FWA units and other wireless communication equipment that utilizes technology wherein a plurality of channels are placed on the frequency axis used in TDD communications technology and many wireless systems. Thereby, it is possible to reduce costs and minimize unit sizes.